Including Discussion of Prior Art
It is well known, that in compression type refrigerating and air-conditioning systems, the compressors employed for compressing the vaporous refrigerant employ oil or lubricant to lubricate their internal parts. In the course of drawing in refrigerant vapor, compressing and discharging the vapor to condensers, some of the lubricating oil is entrained with and discharged with the compressed vapor. Though means for removing some of the oil from the discharge stream are sometimes employed, a small quantity of oil always fails to be removed, traversing such means, and is conveyed with the compressed refrigerant through the condenser and cooling coil or evaporator. The oil leaving the evaporator and entering the suction line must have a way to return to the compressor. Failure to provide such a way can result in accumulation of oil within the refrigerating tubes and pipes and progressive loss of oil from the compressor. It is not uncommon in poorly designed systems for so much oil to be lost from the compressor that insufficient oil is left to properly lubricate and cool the compressor, thereby causing the compressor to fail.
Wherever the compressor is at the same level or lower than the evaporator, oil flow through the vapor return conduit from evaporator to compressor (suction line) is aided by the velocity of vapor flowing through the suction line and by gravity, and the oil returns satisfactorily to the compressor. Even when the compressor is located higher than the evaporator, satisfactory oil return can be simply secured by proper sizing of the upflowing suction line (suction riser) to provide adequate vapor velocity to assure oil return.
However, the compressor-overhead situation is severely complicated when there are several evaporators at various levels below the compressor and the evaporators operate on independent schedules so that the refrigerating loads and therefore the gas velocities through the suction riser vary widely.
One of several strategies found in piping manuals is employed now to cope with this situation. One strategy employs so-called dual risers, where a large and a small riser are coupled together at their bottom and an oil trap is employed to stop flow through the large riser, thereby maintaining satisfactory vapor velocities through the small riser to assure oil return. This arrangement works satisfactorily when the range of loads is small, typically 4:1.
Under this parallel condition the vapor velocity in both must be sufficient to cause oil to flow up the risers. Naturally, great precision and engineering skill is required to properly size the risers and traps. Further, where the loads vary widely, over a range of 10 to 1 or more, such dual riser systems fail to work and oil accumulates in the risers and is lost from the compressor/s. This situation is further complicated and worsened where there are multiple compressors which operate under independent control so that even a single small compressor may run while still requiring satisfactory oil return.
A serious draw back of the dual riser arrangement is that the oil trap removes oils that may be needed for compressor lubrication. When the pressure drop through the small riser becomes so great that it blows out the oil trap, then both risers function together in parallel. Under these blow-out conditions the mass of oil accumulated in the trap may be carried back to the compressor in a slug when the load suddenly increases, thereby raising the possibility of compressor damage from the mass of incompressible oil entering its cylinders.
A further drawback of the dual riser piping arrangement is the difficulty of correctly matching the limited number of pipe or tube sizes available to the required limited range of vapor velocities needed to ensure oil flow up the riser.
A second strategy simply requires that each evaporator have its own suction riser, sized for proper return of oil when the evaporator is operating. This option increases the number of pipes and joints required, thereby increasing the cost of piping and increasing the probability of leaks at the increased number of joint.
Where loads vary very widely and a unitary riser system is desirable, engineers have employed, as a third strategy, oil accumulators are positioned at the bottom of the risers to collect oil which fails to be returned up the riser at conditions of low load and corresponding low suction vapor velocities. This arrangement requires the use of pressure pumps to force the oil collected in the oil accumulator back to the compressor/s through small pipes provided for the purpose. An oil float valve positioned in each compressor crankcase opens when the oil level drops below a predetermined level, thereby causing the oil quantity in that crankcase to be replenished.
The oil return problem is made even more complex where evaporators are so positioned below the compressors that multiple suction risers are required. In that case multiple dual risers may be required further complicating both the engineering and the physical problems and therefore the cost of piping the installation. Where a suction riser, having loads at several levels, is sized for proper oil return with minimum pressure drop, the upper portions of the riser may have a greater diameter than the lower portions. Should one or more the upper loads decrease or drop to zero because its refrigerating or air-conditioning load has been satisfied, oil may rise with the suction vapor in the lower portions of the riser but, because of the decreased load and vapor velocity in the larger upper portions, may fail to rise or be entrained through the upper portions of the riser. Therefore the oil may collect or "log" in the upper portion, thereby causing an oil shortage in the compressor.
The present invention is directed to solving this last problem in a simple manner, without oil pumps and without critical pipe sizing, all while ensuring proper oil return to the compressor over an extremely wide range of full load to minimum load ratios.